I have a good headphone amp in my DAC/preamp, but for portable use I highly recommend my alternate setup, which I run from my phone or laptop. It sounds excellent.
A good set of budget reference phones like my Sennheiser 600 or 650 (Massdrop, now called Drop, does runs of these at a great price). $330 on Amazon Amazon / 650
and...
Audioquest Dragonfly Cobalt DAC/amp